Earlier quoted context omitted.
There's a good chance that if that if Microsoft doesn't act soon enough, and a lot more devices running Steam OS are released, Proton might become the de-facto platform against which many new games are developed, and which engines target. At that point, there is nothing Microsoft can do.
Microsoft controls Windows and DirectX, Valve only gets to play until Windows landlord allows it. DR-DOS, OS/2 and EEE PC. Lets see if SteamOS makes the list as well, this is after all round two, Steam Machines didn't go that well.
So, in a way, the Steam Machines were a great success.
Also, Valve has (for better and worse) far more power and control in the gaming ecosystem than most companies Microsoft has to deal with.